Self-cleaning surfaces of objects and process for producing same

ABSTRACT

The self-cleaning surfaces of objects have an artificial surface structure of elevations and depressions wherein the distances between said elevations are in the range of from 5 to 200 μm, and the heights of said elevations are in the range of from 5 to 100 μm, and at least said elevations consist of hydrophobic polymers or permanently hydrophobized materials, and said elevations cannot be detached by water or by water containing detergents.

The present invention pertains to self-cleaning surfaces of objects anda process for producing same wherein the self-cleaning is preferablyeffected by the surfaces being exposed from time to time to rain ormoving water.

The cleaning of the surfaces of objects is of considerable technical andeconomical importance, partly for optical and aesthetical reasons,partly for technical reasons, particularly in the case of transparentsurfaces which must be cleaned from time to time for preserving theirfunction.

There was no lack of attempts to provide technical surfaces which aredirt-repellent and/or self-cleaning with the aid of rain or movingwater. The success of these efforts has been relatively small, however,since the properties of such surfaces are maintained only for arelatively short period of time. Thus, the manufacturers of polymersheets or polymer boards have tried to solve this problem by producingsurfaces which are as smooth as possible and turning these surfaceseither extremely hydrophobic or extremely hydrophilic. Examples thereofinclude surfaces made of extremely hydrophobic Teflon, or the extremelyhydrophilic “no-drop coatings” from which water and dirt can flow offwithout forming drops.

CH-PS-26 82 58 describes water-repellent surfaces which exhibit acontact angle with water of more than 120°. They are obtained byapplying powders, such as china clay, talc, clay or silica gel, on asubstrate, the powder being first hydrophobized by organic siliconcompounds. The application is performed together with curable resins orfrom solutions in organic solvents. Permanently hydrophobic surfacescannot be produced in this way. Also, there are no indications as to thegrain sizes or grain size distributions of the powders. The propertiesof the surfaces thus obtained are compared with those of the leaves ofnasturtium. With this comparison, it should be noted that it was neitherknown nor technically analyzable whereon the properties of the leafsurface of nasturtium are based. Studies recently performed have shownthat nasturtium has an extremely fine ultrastructure with structuralelements smaller than 2 μm. Such surface structures can hardly begenerated artificially, and moreover are extremely sensitivemechanically. Thus, nasturtium is a very unsuitable model. Therefore,such structures are not the subject matter of the present invention.

DE-PS-10 23 217 describes a mold for the preparation of molded partshaving rough surfaces. The mold is to serve for the preparation ofmolded parts made of rubber or plastic and having rough surfaces. Toachieve this, the walls of the mold are coated with coarse corundumpowder and a stoving paint. The molds yield products having occasionaldepressions and therefore improved adhesive properties. The usualvulcanization skin is even omitted. The surfaces thus obtained are wellinscribable, for example. Thus, the products are surely notself-cleaning with moving water.

JP-A-3-174279 (abstract) describes a process for the preparation ofmatt, decorative surfaces on paper or plastic sheets. They are preparedwith paints which are preliminarily cured by means of ionizing radiationand in which unspecified patterns are stamped in an unspecified way.Then, they are completely cured by further irradiation.

Many years' studies by the Applicant have shown that the surfaces ofcertain plants are capable of cleaning themselves by rain or movingwater and that also no solid dirt particles can be permanently depositedon such surfaces. These dirt-repellent properties have for a long timebeen assigned to various wax layers on the plant surface. However, moredetailed examinations have meanwhile shown that the capability ofself-cleaning of the plant surfaces is not so much dependent on the kindof wax, but on the surface structure of such waxes. Further examinationshave shown that the capability of self-cleaning can in many cases bedestroyed by the use of detergents in the water since this leads to analteration of the wax particles.

The further detailed examinations of the Applicant have now yielded thesurprising result that it is technically possible to artificially renderthe surfaces of objects self-cleaning by artificially providing themwith a surface structure of elevations and depressions, taking care thatthe distances between the elevations of the surface structure are in therange of from 5 to 200 μm, preferably from 10 to 100 μm, and the heightsof the elevations are in the range of from 5 to 100 μm, preferably from10 to 50 μm, and taking care that these elevations consist ofhydrophobic polymers or permanently hydrophobized materials, and takingcare that the elevations cannot be detached by water or by watercontaining detergents.

Such self-cleaning surfaces can be prepared either by creating thesurface structures already in the preparation from hydrophobic polymers,or by creating them subsequently, either by subsequent stamping oretching, or by glueing thereon a powder of the hydrophobic polymers.Finally, it is possible to create such self-cleaning surfaces of objectsby subsequent permanent hydrophobizing of preliminarily preparedsurfaces having the desired structures. One possibility for thesubsequent permanent hydrophobization is subsequent silanization ofpreliminarily prepared surfaces having the desired structures.Silanization can be performed on all materials which are per sehydrophilic, but which are capable of reacting with the reactive groupsof silanes so that the surface ultimately consists of the hydrophobicresidues of the silanes.

Of particular technical importance are self-cleaning surfaces of objectswhich are transparent and which are to retain this transparency for along time for optical, aesthetical or technical reasons. In particular,these are transparent glassworks of buildings, vehicles, sun collectorsetc. Also of economical and technical importance, however, is thepreparation of self-cleaning surfaces of house facades, roofs, monumentsand tents, and of inner linings of silos, tanks or pipelines whicheither contain aqueous solutions or can readily be cleaned by movingwater without leaving any residues. Also of interest are the outercoatings of vehicles, such as cars, trains or airplanes. It is to betaken care then, however, that these surfaces, when cleaned with movingwater, must not be exposed to high mechanical strains since this wouldresult in a leveling or polishing of the surface structures so that theybecome shining, but lose their capability of self-cleaning.

In order to create the desired surface structures in the preparationfrom hydrophobic polymers already, the objects can from the start beprepared in molds which have the negative of the desired surfacestructure. Further, it is possible to apply the hydrophobic polymers inthe form of solutions and/or dispersions which result in the desiredsurface structures when drying and curing. Such structures aregenerated, for instance, from self-organizing polymers, or underconditions as are per se known from the preparation of matt finish paintsurfaces.

If it is not possible or not desired to create the desired surfacestructures from the beginning, this can also be done later, for example,by subsequent stamping or etching. Stamping can be performed, forinstance, by heated or heatable forming punches. Etching can beperformed with the known means of chemical etching, or by physicalmethods, such as ion etching with oxygen or other bombardments whichlead to a roughening of the surface and thus to a suitable surfacestructure according to the invention.

Further, it has been shown that it is also possible to obtain thedesired surface structure by glueing a powder of the hydrophobicpolymers. Powders of hydrophobic polymers having the desired grain sizesare available. Optimum results are only achieved, however, if powdersare employed having a relatively narrow grain size distribution.

It could not have been foreseen that the surface structures according tothe invention having elevations and depressions consisting ofhydrophobic polymers or corresponding subsequently permanentlyhydrophobized surface structures are capable of being cleaned by rain ormoving water without leaving any residues. In particular, it had to beexpected that solid particles which are smaller than the distancesbetween the elevations would lead to an occupation of the interspacesand thus reduce the desired self-cleaning properties. It has been found,however, that the self-cleaning effect will function perfectly, even ifthere is contamination with particles so extremely fine that they becomenested in the interspaces between the surface sculptures: impingingwater drops (e.g., rain) are briefly pressed between the microsculpturesin the very moment of impinging due to their kinetic energy, and dirtwhich is sticking there is torn away and subsequently washed offtogether with the rolling drop.

Further, it has been shown that the self-cleaning surfaces according tothe invention may positively even come into contact with watercontaining detergents. The self-cleaning property of the surfaces isfirst lost, but will be regained as soon as the detergents are removedagain by subsequent raining or washing with pure water. Thus, incontrast to the self-cleaning plant surfaces, the detergents cannotcause permanent damage. Thus, the self-cleaning property of the surfacescan only be destroyed by mechanical destruction of the surfacestructures. According to the invention, it is possible even then,however, to restore the self-cleaning effect at least to a certainextent by glueing a suitable powder of hydrophobic polymers. Evidently,the water-repellency is based on the fact that water drops rest only onthe tops of the elevations and thus have only an extremely small contactarea with the surface. However, relatively much air is trapped betweenthe water drops and the surface of the depressions so that the interfaceappears silvery. Since air is a very hydrophobic material, the dropadopts the smallest possible surface, i.e. it becomes a sphere and thusrolls off the surface upon the least vibration. In a similar way, theadhesion of solid particles to the surface is also reduced. Irrespectiveof their chemical nature, they exhibit a more or less high affinity towater drops so that they are removed from the surface together withdrops rolling off.

Optimum results are achieved when the elevations of the surfacestructures are sufficiently close to one another to avoid a contact ofwater drops with the depressions lying between the elevations. If theelevations of the surface structures are too close to one another or thedepressions are not deep enough, they again act as a closed surface andthus can be more readily wetted. Therefore, it should be sought thatwith increasing distance of the elevations, the heights of theelevations above the ground should also increase. The measurementsperformed to date have shown that within the claimed limits of thedistances and heights of the elevations, good results are achieved.Optimum results are yielded by surfaces having elevations of from 10 to50 μm in height wherein the distance between the elevations is from 10to 100 μm. At any rate, such surface structures can be used wherever aself-cleaning by moving water, such as rain, is desired withoutdetergents.

In the following examples, preferred embodiments on a laboratory scaleare described. However, methods similar in principle can be used on alarge, industrial scale, including those which have already been usedfor other purposes and with other dimensions for altering and designingsurfaces without generating the surfaces according to the inventionhaving self-cleaning properties.

EXAMPLE 1

A smooth surface made of a plastic material, such as Resopal orpolyethylene, is uniformly coated with a thin layer of a glue, such asUHU PLUS®, and subsequently coated with a Teflon powder, such asHostaflon® TF 9205 (average particle size 7 μm). After curing, a surfaceis formed from which deposited particles, such as carbon black andpowdered pigment, can be washed with water.

EXAMPLE 2

A smooth hydrophobic material, such as PTFE, is heated until it becomesplastic in character. Then, a high mesh screen from offset printing ispressed onto the surface and removed again. After cooling, a surface hasformed having regularly arranged elevations and depressions ofcomparable heights. By using different screens having different meshwidths and thicknesses, the dimensions can be changed and optimallyadjusted. The properties of the surfaces thus obtained are optimum whenthe elevations have rounded tops of course, such surface structures canalso be prepared by means of heated punches or rolls. Correspondingsheets can be adhered to another, smooth substrate.

What is claimed is:
 1. An object having an exposed surface, said exposedsurface consisting essentially of elevations and depressions, whereindistances between elevations are in the range 5-200 μm, heights of saidelevations are in the range 5-100 μm, said elevations are made ofhydrophobic polymers or permanently hydrophobized materials, saidelevations cannot be detached from said object by water or watercontaining detergents, and said exposed surface is self-cleaning uponexposure to rain or running water, alone, in that drops of water on thesurface become substantially a sphere and solid particles on the surfaceare removed from the surface together with the drops of water rollingoff the surface.
 2. The object of claim 1, wherein distances betweenelevations are in the range 10-100 μm, and heights of said elevationsare in the range 10-50 μm.
 3. The object of claim 1, wherein distancesbetween elevations are in the range 10-100 μm.
 4. The object of claim 1,wherein heights of said elevations are in the range 10-50 μm.
 5. Theobject of claim 1, wherein said exposed surface is transparent.
 6. Amethod of affecting surface properties of an object comprising the stepof forming on said object an exposed surface consisting essentially ofelevations and depressions such that distances between elevations are inthe range 5-200 μm, heights of said elevations are in the range 5-100μm, said elevations are made of hydrophobic polymers or permanentlyhydrophobized materials, said elevations cannot be detached from saidobject by water or water containing detergents, whereby, said exposedsurface is self-cleaning upon exposure to rain or running water, alone,in that drops of water on the surface become substantially a sphere andsolid particles on the surface are removed from the surface togetherwith the drops of water rolling off the surface.
 7. The method of claim6, wherein distances between elevations are in the range 10-100 μm, andheights of said elevations are in the range 10-50 μm.
 8. The method ofclaim 6, wherein distances between elevations are in the range 10-100μm.
 9. The method of claim 6, wherein heights of said elevations are inthe range 10-50 μm.
 10. The method of claim 6, wherein said exposedsurface is transparent.